In a display market, a liquid crystal display rapidly has enlarged its territory and replaced CRT because it has some merits (better properties) such as low power consumption, small size especially in thickness, and lightness.
Referring to the early stage products of a liquid crystal display, a reflection type was the most common type. So the external light source to light the display panel was required. However, the reflection type liquid crystal displays of the early stage hardly meet the requirement for high quality, multi-media display in the respects of correspondence time, color appearance, efficiency of reflection, and contrast ratio.
Research and development on the other types of liquid crystal display such as PCGH (Phase Change Guest Host) type, PDLC (Polymer Dispersed Liquid Crystal) type, PSCT (Polymer Stabilized Cholesteric Texture) type are also done. However, they have other types of problems such as high operation voltage and difficulties in gray scale display.
In comparison with above-mentioned types of a liquid crystal display, the MTN (Mixed Twisted Nematic) mode which is being developed nowadays has its own strong point in productivity and in reliability. Further, the MTN mode has merits in reflection efficiency, contrast ratio, and color appearance. The MTN mode is a kind of TN mode having retardation film to compensate for phase difference caused by liquid crystal layer in the path of light.
But, in the present Window environment, normally dark MTN mode causes increase in power consumption and lacks of cell gap uniformity. Moreover, according to the optical property of LC, if a wave length of light goes further to an infrared region, the more the intensity of light in dark mood increases. The increase of light intensity in dark mode worsens the contrast ratio of a liquid crystal display.
To solve such a problem in the MTN mode, the method of differentiating cell gap of pixel to the color RGB (Red, Green, Blue) can be considered. But, it is hard to make a liquid crystal display panel having pixels of different cell gap in the color.
Therefore, another method is usually adopted, which uses a retardation film to operate a liquid crystal display in a normal white mode.
FIG. 1 shows a basic structure of a panel substrate in an MTN mode reflection type liquid crystal display and optical bases of normal white mood operation in the liquid crystal display. In FIG. 1 and hereinafter, the electrodes applying an electric field to a liquid crystal layer are omitted.
By these figures, relationship of composing elements in the structure of a liquid crystal display and phase change of light passing through the elements can explicitly be shown.
Let a vertical line in the FIG. 2 be a z-axis, a horizontal line an x-axis. At this time, a line perpendicular to the figure plane is a y-axis. The incident light having the direction of a positive z-axis (i.e., downward in the figure) is not polarized so that the incident light can vibrate in all directions of the x-y plane.
First, in case of a white mode, the light incident on the panel is transformed to lineally polarized light by a polarizer 11 which is generally attached to a surface of a front (upper) substrate glass. The polarized light vibrates in the x-direction that is the direction of a transmissive axis of the polarizer 11.
The slow axis of a retardation plate 13 makes angle of −45′ in view of clockwise circulation with the transmissive axis of the polarizer 11. So the lineally polarized light is changed into circularly polarized light which circulates counter clockwise by passing through the retardation plate 13. If there is no electric field in a liquid crystal layer 19, the liquid crystal layer 19 is twisted. Generally, the angle is 90 and corresponds to one-forth of the wavelength of a passing light in an MTN mood liquid crystal display. The circularly polarized light passing through the liquid crystal layer 19 is changed into lineally polarized light again. At this time, however, the lineally polarized light vibrates only in y-axis direction.
Then, the light is reflected on a reflector 41 placed on the inner surface of a rear (lower) substrate glass. In the reflection, phase shift of the lineally polarized light corresponds to double of right angle and thus maintains the y-axis vibration, practically the same state.
The reflected light passes through the liquid crystal layer 19 and the retardation plate 13 again. The phase of the reflected light is reversely changed, so that the reflected light is circularly polarized light rotating counter-clockwise and is lineally polarized light vibrating only in an x-axis direction then. The polarized light vibrates in the x-direction that is the direction of the transmissive axis of the polarizer 11. As a result, the reflected light finally passes the polarizer 11 without serious decrease in intensity.
If a panel of liquid crystal display is operated in this mode, the liquid crystal display panel looks bright when no electric potential is applied to the liquid crystal layer. So this type of a liquid crystal display has merits in view of less electric energy consumption in a normal white windows operation system. In a color type liquid crystal display possessing color filter, although the light passing each pixel of the color filter shall have its own color, the total effect of the light passing through some broad area of the color filter is represented by white color, substantially the same to black-white type.
Next, in case of a dark mode, incident light to the liquid crystal display panel goes through the same pass with same phase change before the light reaches the liquid crystal layer 19. When the light reaches the liquid crystal layer 19, the light is circularly polarized and the liquid crystal molecules are arrayed in parallel and upright because of the applied electric field. Thus, circularly polarized light undergoes no phase shift in the proceeding and maintains the same phase state.
Then, the circularly polarized light is reflected at the reflector 41 placed on the inner surface of the lower substrate glass. By the reflection, the phase shift of the light makes the reflected light be orthogonal to the incident light to the reflector 41. The reflected light now becomes circularly polarized light circulating clockwise, passes the liquid crystal with no phase shift, and reaches the retardation plate 13.
The retardation plate makes the circularly polarized light be polarized light vibrating toward only y-axis which is perpendicular to the direction of the transmissive axis of the polarizer 11. As a result, the reflected light hardly passes the polarizer.
If a liquid crystal display panel is operated in this mode, the panel looks dark in the state of applied potential.
In the above description, it is expected that a retardation film serves as an ideal λ/4 plate to all the range of visible light (380 nm-780 nm). However, all of the real materials in retardation film almost have wavelength dispersion characteristics, as shown in FIG. 2. Therefore, a value of delta nd becomes smaller to light of farther infrared region. If the retardation plate is designed to accurately serve as visible light of middle-ranged wavelength, for the light of shorter wavelength in a visible region, the intensity of reflected light is strengthened in dark mode. A retardation film of one sheet cannot make an ideal λ/4 plate to the whole range of visible light. In reflection type liquid crystal display, according to the wavelength dispersion character, some part of incident light re-penetrates the polarizer and goes from the liquid crystal display panel after polarization and reflection. Further, the penetrated light cuts contrast ratio of a liquid crystal display and degrades quality of the liquid crystal display.